The control system of an automatic transmission is typically hydraulically operated through the use of valves which direct and regulate the supply of pressure. This hydraulic pressure control causes either actuation or deactuation of frictional members for affecting gear changes in the transmission. Traditionally, such valves have comprised spring-biased spool valves, spring-biased accumulators, and ball check valves.
These spring-type valves prove problematic in certain areas, primarily because they were limited to use in a narrow range of engine designs. More recently, automatic transmission designs have included adaptive control systems which utilize electrically operated solenoid-actuated valves for controlling various fluid pressures.
However, current solenoid-actuated valve designs also have their shortcomings. For example, vibrationally-transmitted impact noise during shifting results in a chattering effect from solenoid actuation. This chattering is a result of the pulse width modulated (PWM) electronic drive signal which actuates the solenoids. This occurs under step shifting conditions in which the solenoid ramps from 0 to 100 percent duty cycle or vice versa. The vibrational energy generated is at the excitation PWM frequency, as well as the higher order multiple of the drive frequency. This vibrational energy directly transmits from the solenoid through its mounting interface into the transmission housing.
Another problem with current designs is airborne noise from valve actuation. Certain current designs include an elastomeric sound shield mounted over the module when installed on the vehicle transmission. In order to retain the sound shield over the module, a spacer plate along with an extra gasket are required. These added components increase cost to the customer in both material and labor, while providing an added external leak path. Furthermore, the sound shield is not particularly effective in airborne noise abatement.
Another problem inherent with current designs is that all solenoid components must be completely assembled prior to testing. Because of this, it is difficult to use "poka-yoke" techniques in sub-assemblies upstream to prevent problems from occurring downstream, which are typically detected only when the module is fully assembled. At this point, it is common to disassemble and rework a significant number of solenoid modules due to problems which could have been detected prior to assembly completion.
A further problem experienced with current designs is particulate sensitivity. The transmission fluid circulating about the valve includes particulate matter which tends to attract to the armature and pole piece, particularly ferrous particles. This build-up of particulate matter can become great enough to diminish the effectiveness of the armature or to completely disable the valve, which results in a default condition.